In many oil and gas wells where cementing casing in an existing borehole is required, a cement column must be placed from the bottom of the well to or near the surface. The strength of the formation (rock) may not allow such placement in a single pumping program. In such cases, a multiple stage cementing program must be achieved.
In order to achieve more than one circulation of cement, devices are provided to open and close a hole in the casing. Such devices known as “stage tools” are operated between the open and closed positions using hydraulic forces, including plugs displaced from the surface to the tool. Port collars serve a similar purpose, and are opened by a mechanical shifting device on a tubular string (work string) inside the casing.
In addition to providing the device to open and close a hole in the casing, many applications also require the use of a packer to seal the annulus between the casing and open hole to support the additional hydrostatic pressure which will be exerted by the higher density cement slurry when placed into the annulus. Devices used to achieve this seal are commonly known as casing packers, and may consist of an inflatable device or compression seal device, each activated by the application of fluid pressure from the inside of the casing to the expandable packer element.
Typically, inflatable packers contain two or more valves in a packer head. One valve normally controls the pressure at which inflation is initiated, and a second valve controls the maximum pressure that is applied to the packer. In cases where the inflatable packer element ruptures during the inflation process or afterwards, these valves are designed to fail in a position which does not leave a flow path between the inside of the casing and the annulus.
Due to varying conditions in wells, these fail safe valves do not always function properly and may require remedial operations to eliminate the flow path, such as squeezing cement into the valve ports. Such operations can be expensive but must be successful in order to continue drilling deeper or completing the well for production of oil or gas. Similar valves in compression seal packers have the same shortcomings, and may similarly require a cement squeezing operation to close off flow to the annulus.
A standard procedure for achieving a two-stage cementing program is to pump a volume of cement down the inside of the casing and out the end. The volume pumped is determined by the capacity of the formations to withstand the additional hydrostatic pressure applied by the cement column without fracturing or otherwise causing the cement to penetrate into the formations. When such second stage cementing operations are achieved by using a “stage tool,” any cement left inside the casing and the displacement plugs that activate opening or closing of the ports must be removed by drilling through the tool, since no circulation is possible once the packer is set. Drilling cement and plugs from the inside of a stage tool can be difficult, particularly at shallow depths where there may be minimal weight of the pipe string used for such drilling, so that drilling penetration rates through the cement and plugs are slow. Hydraulically operated stage tools also require a large cement pumping volume due to the diameter of the casing, and hydraulic plugs may have sealing reliability problems.
Particularly in wells where the second stage application is shallow, a few hundred feet for example, the port collar device is preferred as such is opened, closed and tested by a mechanical shifting device run on a tubular, such as drill pipe. Any excess cement left in the tubing or casing may be removed by circulation between the interior of the casing and the interior of the drill pipe.
Relevant patents include U.S. Pat. Nos. 1,684,551, 2,435,016, 2,602,510, 2,659,438, 2,928,470, 3,247,905, 3,464,493, 3,503,445, 3,527,297, 3,948,322, 4,424,860, 4,479,545, 4,499,947, 4,850,432, 5,024,273, 5,109,925, 5,297,633, 5,314,015, 5,375,662, 5,383,520, 5,488,994, and 5,400,855.
The disadvantages of the prior art are overcome by the present invention, and an improved packer with a controlled port collar is hereinafter disclosed.